This invention relates to a circuit for de-stabilizing a servo loop, and more particularly to such a circuit for automatically adjusting servo loop gain during a programmed period of instability so that a sufficiently high amplitude, low frequency of oscillation may be obtained to which the servoed member in the servo loop will respond.
There are a number of circuits and methods for imposing what is termed "dither" in the servoed member of a servo loop for the purpose of removing zero "offset" resulting from the effect of static friction in the supports for the servoed member. One such apparatus and method is disclosed in U.S. Pat. No. 3,797,320 owned by the assignee in this application. The electronic dither circuit disclosed there destabilizes a servo loop allowing the servoed member to oscillate about a reference position by removing the damping in the servo loop through short-circuiting the feedback circuit which controls gain and which provides servo loop stability. The short circuit is obtained through the use of a photocell in conjunction with a light emitting diode. The light emitting diode is caused to emit light energy which impinges on the photocell, and thereby drives the resistance of the photocell towards a zero value. The light emitting diode is energized for a predetermined period of time by a timing circuit in response to a "start-up" signal. The disclosure of the above referenced patent mentions at column 6, lines 60 et seq., that the photo resistors serve as a variable control element and may be replaced by field effect transistors or other semi-conductor devices. As a matter of practical fact, it has been found that field effect transistors do not possess symmetrical resistance characteristics for large alternating voltage swings between the source and drain terminals when an enabling signal is present at the field effect transistor gate. As a consequence, field effect transistors, when turned on in place of the photocell, will provide unequal servo loop gain during servo loop oscillation, and will therefore tend to drive the moving or servoed member away from the reference position, thereby leaving a large zero offset at the end of the unstable period, which can be equal to or greater than the offset due to static friction which was to be removed by the oscillation.
A circuit is therefore necessary in which the circuit component utilized to impose the servo loop instability does not impart an asymmetrical amplitude to the servo moving member during oscillation or inject a potential zero offset error equal to or greater than that which was intended to be removed.